Peripheral vascular array

ABSTRACT

An interface that couples a surface coil array having N receiving elements to an NMR scanner having M preamplifier inputs. The interface includes an RF switch array and a transmit/receive bias circuit. The RF switch array and the transmit/receive bias circuit are controlled by a control logic circuit. In response to a predetermined input, the control logic circuit causes a predetermined subset of the N receiving elements to be couple to the preamplifier inputs of the NMR scanner. Preferably, N may be larger than M.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to nuclear magnetic resonance(“NMR”) imaging and, more particularly, to methods and apparatus forimaging the peripheral vasculature.

[0002] Initially, NMR imaging systems utilized receiver coils whichsurrounded the entire sample (for example a human patient) that was tobe imaged. These remote coils had the advantage that the sensitivitywas, to a first approximation, substantially constant over the entireregion being imaged. While this uniformity in sensitivity is notstrictly characteristic of such remote coils, the sensitivity issubstantially constant to a sufficient degree that most reconstructiontechniques assume a constant coil sensitivity. Because of their largesize the remote coils suffer from a relative insensitivity to individualspins.

[0003] For certain applications, a surface coil is preferable to aremote coil. Surface coils can be made much smaller in geometry thanremote coils and for medical diagnostic use can be applied near, on, orinside the body of a patient. This is especially important whereattention is being directed to imaging a small region within thepatient, rather than an entire anatomical cross section. The use of asurface coil also reduces the noise contribution from electrical lossesin the body, with respect to a corresponding remote coil, whilemaximizing the desired signal. NMR imaging systems thus typically use asmall surface coil for localized high-resolution imaging.

[0004] A disadvantage of the surface coil, however, is its limited fieldof view. A single surface coil can only effectively image that region ofthe sample having lateral dimensions comparable to the surface coildiameter. Therefore, the surface coil necessarily restricts the field ofview and inevitably leads to a tradeoff between resolution and field ofview. The size of the surface coil is constrained by the intrinsicsignal to noise ratio of the coil. Generally, larger coils inducegreater patient sample losses and therefore have a larger noisecomponent, while smaller coils have lower noise but in turn restrict thefield of view to a smaller region.

[0005] One technique for extending the field-of-view limitation of asingle surface coil is described in U.S. Pat. No. 4,825,162 to Roemer etal. Roemer et al. describes a set of surface coils arrayed withoverlapping fields of view. Each of the surface coils is positioned soas to have substantially no interaction with all adjacent surface coils.A different NMR response signal is received at each different one of thesurface coils from an associated portion of the sample enclosed withinan imaging volume defined by the array. Each different NMR responsesignal is used to construct a different one of a like plurality of NMRimages of the sample, with the plurality of different images then beingcombined to produce a single composite NMR image. Roemer et al.describes a four-coil array for imaging the human spine.

[0006] While an increased number of surface coils may be used toincrease the field of view, NMR system scanners typically have a limitednumber of preamplifier input. The number of preamplifier inputs istherefore a design limitation in the design of phased array surfacecoils. A disadvantage of known phased array surface coils, therefore, isthat the surface coil array may include only as many coils as can bedirectly connected to the preamplifiers of the system scanner.

[0007] One technique for constructing images of areas of greater sizefrom the limited filed of view of known surface coil combinations is tomove the surface coils after successive scans. This technique however,requires excessive scan room intervention. That is, after each scan, atechnician enters the scan room to physically re-position the coils.This may increase examination time and increase the likelihood of apatient rejecting the procedure.

[0008] It would be desirable to obtain increased field of view withoutscan room intervention.

[0009] It would also be desirable to have an improved phased arraysurface coil for providing a large field of view. It is furtherdesirable to utilize a greater number of surface coils in the array.

SUMMARY OF THE INVENTION

[0010] In accordance with a first aspect of the present invention, acoil interface is provided. The coil interface includes N coils forsensing image signals and a number of switches connected to the N coils.The coil interface also includes circuitry for selecting a group of theN coils. The selection may be made by enabling a selected group of thenumber of switches in response to a group selector input. The coilinterface further includes a number. M, of outputs to an NMR scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic design of a system for receiving an NMRresponse signal in accordance with a preferred embodiment of the presentinvention.

[0012]FIG. 2A and 2B are schematic representations of a peripheralvascular array that is operable with the system of FIG. 1.

[0013]FIG. 3 illustrates an embodiment of a housing for the peripheralvascular array shown in FIGS. 2A and 2B.

[0014]FIGS. 4A, 4B and 4C schematically illustrate the capability of theperipheral vascular array housing shown in FIG. 3 to accommodate avariety of body types.

[0015]FIGS. 5 through 16 are electrical schematic diagram of the surfacecoils in the peripheral vascular array shown in FIGS. 2A and 2B.

[0016]FIG. 17 is a block diagram of an NMR scanner and a 20-coil surfacecoil array that uses an interface in accordance with a preferredembodiment of the present invention.

[0017]FIG. 18 is a coil group table showing groups of surface coils, amode switch setting, surface coils selected by a particular group andcomments regarding an image obtained using the selected group of surfacecoils.

[0018]FIG. 19 is an electrical schematic of the T/R driver shown in FIG.17.

[0019]FIG. 20 is an electrical schematic of the RF switch array shown inFIG. 17.

[0020]FIG. 21A through 21C are electrical schematics of the RF switchesshown in FIG. 20.

[0021]FIGS. 22A and 22B are electrical schematics of a preferredimplementation of the RF switch array shown in FIG. 20.

[0022]FIG. 23A illustrates a programmable logic device in a preferredimplementation of the control logic shown in FIG. 17.

[0023]FIGS. 23B and 23C are state tables for the control logic shown inFIG. 17.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0024]FIG. 1 is a schematic diagram of a system for receiving an NMRresponse signal in accordance with a preferred embodiment of the presentinvention. The system includes a surface coil array 10 that is connectedby an interface 20 to an NMR scanner 30. The surface coil array 10includes a number, N, of surface coils 12. Each of the surface coils 12is electrically connected through a transmit/receive (“T/R”) biascircuit 21 to an RF switch/combiner 22 in the interface 20. The RFswitch/combiner 22 has a plurality of outputs 23 that are connected to aplurality of receiver preamplifiers 32 in the NMR scanner 30.

[0025] The interface 20 also includes a control logic circuit 24, whichis coupled to and controls the RF switch/combiner 22 and the T/R bias21. The control logic circuit 24 has three inputs. The first input is aDC power input 26, which is provided by a power supply 34 in the NMRscanner 30. The second input is a coil select input 28. The third inputis a mode select input 25. The control logic circuit 24 selectivelyactivates a predetermined arrangement of surface coils 12 in accordancewith the coil select input 28 and the mode select input 25.

[0026] As shown in FIG. 1, the coil select input 28 originates from theNMR scanner 30. However, the coil select input 28 may alternatively beprovided to the control logic circuit 24 from another source that isexternal to the interface 20, as illustrated by the dashed coil selectinput 29 in FIG. 1. The coil select input 28 is preferably a DC controlsignal. Blocking capacitors may therefore be used at the outputs 23 ofthe RF switch/combiner 22 to block DC from the RF switch 22. In asimilar manner, RF chokes may be used on the coil select input 28 toprevent RF signals from reaching the control logic circuit 24. Thedesign of DC blocks and RF chokes are well known to those of ordinaryskill in the art. When the coil select input 28 originates from the NMRscanner 30, it may be superimposed on some or all of the outputs 23. Asshown in FIG. 1, four outputs 23 are used, such that the coil selectsignal 28 may be treated as a 4-bit word.

[0027] In addition, as shown in FIG. 1, the DC power input 26 isprovided by the power supply 34 in the NMR scanner 30. However, the DCpower input 26 may alternatively be provided by a power supply, such asa battery, contained within or connected to the interface 20.Preferably, the battery is constructed of materials that are notresponsive to and do not adversely effect the magnetic fields in the NMRexamination room. For example, the battery may be a rechargeablelead-acid battery with a gel electrolyte and a plastic housing, such asthe Gel Cell™ batteries that are commercially available from Globe. As astill further alternative, the DC power input 26 may be provided by anyDC source that is external to the scanner 30. These alternatives areparticularly appropriate when the scanner 30 either does not provide aDC power supply output or provides a DC power supply output that isinsufficient to power the interface 20 and the array 10.

[0028] During imaging, the surface coils 12 that are activated to thereceive state produce RF signals that are coupled to the RFswitch/combiner 22. The outputs 23 of the RF switch/combiner 22 are thencoupled to preamplifiers 32 in the NMR scanner 30. The operation of theinterface 20 is described in greater detail below with reference toFIGS. 17 through 22.

[0029] The scanner 30 includes a predetermined number, M, of thereceiver preamplifiers 32. In accordance with a preferred embodiment ofthe present invention, the interface 20, as described below, allows asurface coil array 10 having a greater number, N, of surface coils 12than the number, M, of receiver preamplifiers 32, to be connected to thescanner 30 (i.e. N≧M).

[0030] In accordance with a preferred embodiment, the array 10 is areceive-only, phased array surface coil and the NMR scanner 30 iscapable of operating in a phased array receiving mode. Such NMR scannersare commercially available. For example, the Signa™ family of magneticresonance imaging systems, equipped with phased array capability, areavailable from GE Medical Systems, Inc. of Waukesha, WI. These scannersare designed to accept up to eight preamplifier inputs (M=8).

[0031] In accordance with a preferred embodiment of the presentinvention, the surface coil array 10 is a peripheral vascular array. Theperipheral vascular array is useful for studies relating to peripheralvascular disease. These studies may include deep vein thrombosisscreening, aortic runoff studies, distal vessel patency, thedetermination of the existence location, length, and severity ofstenoses, and the search for patent distal vessels that are suitable forbypass grafts. The peripheral vascular array is therefore preferablycapable of imaging vasculature from the area of the kidneys anddescending through the lower extremities to the feet. Because of thislarge field of view, the peripheral vascular coil may also be useful forapplications involving soft tissue imaging, such as screening formetastic disease, and long bone imaging.

[0032]FIGS. 2A and 2B show a schematic representation of a peripheralvascular array 40 in accordance with a preferred embodiment of thepresent invention. In FIG. 2A, an arrangement of the surface coils inthe array 40 is illustrated. The peripheral vascular array 40 includesten posterior coils, 42 through 51, and ten anterior coils, 52 through61. Each of the coils 42 through 61 is a surface coil that receivessignals from hydrogen protons during NMR imaging.

[0033] Referring again to FIG. 2A, surface coils 42, 43, 52 and 53 aresingle loop coils. Surface coils 44 a, 44 b and 45 a, 45 b arecounter-rotating loops. Surface coils 54 a, 54 b and 55 a, 55 b areco-rotating loops. The surface coils in the lower leg section, surfacecoils 46-51 and 56-61, are single loops when operated in unilateralmode, as described below. In bilateral mode, which is also describedbelow, surface coil pairs (46, 47), (48, 49) and (60, 61) are combinedas co-rotating loops and surface coil pairs (50, 51), (56, 57) and (58,59) are combined as counter-rotating loops.

[0034]FIG. 2B illustrates how the surface coils shown in FIG. 2A may bearranged about the patient to obtain images of the vascular structuresof the abdomen, pelvis and lower limbs. In particular, the peripheralvascular array 40 may obtain images of the vascular structures from therenal arteries through the feet without moving the patient or the array40. Thus, the peripheral vascular array 40 advantageously allows a largeregion of the patient to be imaged without requiring scan roomintervention by an operator. This may decrease examination times andminimize the likelihood of patient rejection.

[0035] As shown in FIGS. 2A and 2B, with the exception of the surfacecoils 42, 43 and 52, 53, where an anteriorly located surface coiloverlies a posteriorly located coil, one of the coils is of theco-rotating type and the other is of the counter-rotating type. Inaddition, where adjacent surface coils in the array 40 may significantlyoverlap, such the surface coil 45 a, 45 b and the surface coil pair 46,47, one of the coils is preferable of the co-rotating type (e.g. pair46, 47) and the other is of the counter-rotating type (45 a, 45 b). Thisalternation between co-rotating and counter-rotating structures providesthe benefit of improving the isolation between the coils, whoseintrinsic isolation is then maintained even if the vertical spacingbetween the coils changes, the superior/inferior offset between opposingcoils changes, or the superior/inferior offset between adjacent coils isadjusted. The superior/inferior offset between adjacent coils may beadjusted, for example, by telescoping the surface coils 45 through 51and 56 through 61 toward or away from the surface coils 45 and 55.

[0036]FIG. 3 shows an exploded view of a housing 62 for the peripheralvascular array 40. The housing 62 is constructed to position the coilsof the peripheral vascular array 40 as shown in FIG. 2B. The housing 62includes a tray 64 that is constructed to support the legs of thepatient. The tray 64 has a recess 66 at its distal end. At the oppositeend of the tray 64 from the recess 66, an incline 68 is formed in theupper surface of the tray 64. A lumbar support 70 extends from the tray64 at an edge 72 adjacent to the incline 68. The lumbar support 70includes a support surface 74 and a positioning member 76. Thepositioning member 76 fixes the relative portion of the lumbar support70 and the tray 64. In the alternative, the positioning member 76 mayallow the lumbar support 70 to be extended from or drawn closer to thetray 64 in order to accommodate patients of varying size. The positionmember 76 extends into the tray 64 at the edge 72.

[0037] The housing 62 also includes a leg support structure 78. A topportion 80 of the leg support 78 is attached to a bottom portion 82 by acoupler 84. The coupler 84 preferably allows the position of the topportion 80 to vary with respect to the bottom portion 82. The legsupport 78 is slidably mounted within the recess 66 in the tray 64 sothat the upper surface of the bottom portion 82 is flush with the uppersurface of the tray 64. Because the leg support 78 is slidably mounted,it may be moved to accommodate variations in patient size.

[0038] The housing 62 further includes a cover 86. A first end 88 of thecover 86 is shaped to fit over the top portion 80 of the leg support 78.In this manner, the cover 86 may slide over the top of the leg support78 when the leg support 78 is moved along the recess 66. A flexibleextension 90 protrudes from a second end 92 of the cover 86.

[0039] The location of the surface coils within the embodiment of thehousing 62 that is shown in FIG. 3 will now be described with referenceto FIGS. 2A and 2B. The tray 64 includes posterior surface coils 44 and45, with the surface coil 44 being substantially located below thesurface of the incline 68 and the coil 45 being located below thesurface

[0040]FIGS. 4A, 4B and 4C schematically illustrate the flexibility ofthe peripheral vascular array 40 in accommodating a variety of bodytypes. As shown in FIGS. 4A through 4C the peripheral vascular array 40is advantageously able to maintain imaging coverage from the renalarteries through the feet whether the patient is of relatively smallstature, such as in FIG. 4A, or large stature, such as in FIG. 4C. Thisis advantageous because patients afflicted with peripheral vasculardisease are frequently significantly larger or smaller than the averageperson.

[0041] The top portion 80 of the leg support 78 and the cover 86 arepreferably constructed in a lattice type framework. This reduces theweight of the coil, making it easier for the technician to use while atthe same time improving patient comfort by allowing air to flow aroundthe patient to enhance cooling. In addition, the housing 62 allows thepatient's arms to remain unrestricted, thereby reducing claustrophobicreactions that are sometimes experienced by patients who are subjectedto the close confines of NMR scanners. Moreover, once the peripheralvascular array 40 is adjusted to accommodate the size of the patient, nopatient or coil movement is required to complete the examination. Thisreduces examination times and increases patient comfort.

[0042] The surface coils 42-61 are typically formed from copper traceshaving a thickness of 0.0028″ and a width of 0.5″. Copper bars or tubingmay alternatively be used as coil conductors. The peripheral vasculararray 40 preferably contains a practical minimum of conductivematerials. This will aid in the reduction of eddy currents at thefrequencies corresponding to NMR gradient coil wave forms, thusminimizing the possibility of artifacts. In addition, the peripheralvascular array 40 preferably contains a practical minimum offerro-magnetic materials to minimize the interaction of the array 40with the B0 main magnetic field of the host NMR system.

[0043] The tray 64 is preferably made from ABS using a vacuum formingprocess. The surface coils 44 and 45 are then adhered to the bottom ofthe upper surface of the tray 64. The bottom portion 82 of the legsupport 78 is preferably constructed using a low-pressure polyurethaneresin to encase the surface coils. Flexible areas of the housing 62,such as the flexible extension 90, are formed by sandwiching the surfacecoils between foam and then encasing the foam in fabric cover.

[0044]FIGS. 5 through 16 are electrical schematic diagrams of thesurface coils 42 through 61. Each of the surface coils 42 through 61preferably includes a PIN diode. D, for switching the surface coils 42through 61 between the receive state and an active disabled state. Thisprovides the advantage of decreasing undesirable coil interaction thatmay reduce image quality, particularly during unilateral imaging. As isknown in the art, the surface coils are preferably actively disabled byPIN diode switches during the RF transmit state. In addition, FIGS. 5through 16 show passive blocking networks 96. The passive blockingnetworks 96 assist the PIN diode switches in disabling the surface coils42-61 during the RF transmit state.

[0045] Furthermore, FIGS. 7 through 16 show implementations of networks94, including component values, for isolating adjacent coils to furtherimprove image quality by reducing shading and aliasing artifacts. Thenetworks 94 in FIGS. 7 through 16 also perform the matching andswitching functions. As shown in FIGS. 5 and 6, the mutual inductancebetween surface coils 42 and 43 and between surface coils 52 and 53 isreduced by overlapping adjacent coils in a manner that is known to thoseskilled in the art although isolation networks may alternatively beused.

[0046] Referring now to FIG. 5 , an electrical schematic diagram for thesurface coils 42 and 43 is provided. Each surface coil 42 and 43includes a passive blocking network 96, loop capacitances, and an inputnetwork 98. The input network 98 includes the PIN diode, D,, blockingand matching elements, and a 50-Ohm lattice balun 100. The balun 100,which is also shown in FIGS. 6 through 16 (although with differingcomponent values), suppresses common-mode currents. Component values forthe elements shown in FIG. 5 are as follows: Loop Components Balun 100Input Network 98 C₁, C₅ = 1-16 pf L₂, L₃ = 142 nh C1 = 91 pf C₂ = 75 pfC₃, C₄ = 51 pf C2 = 100 pf C₆ = 91 pf D1 = UM 9415 PIN diode C₃, C₄ = 82pf L1 = 92 nh C₇, C₈ = 82 pf C₉, C₁₀ = 82 pf

[0047] Diodes=Unitrode Diodes

[0048]FIG. 6 is an electrical schematic diagram for surface coils 52 and53 Component values for the elements shown in FIG. 6 are as follows:Loop Components Balun 100 Input Network 98 C₁, C₂ = 51 pf L₂, L₃ = 142nh C1 = 56 pf C₃, C₅ = 47 pf C₃, C₄ = 51 ph C2 = 100 pf C₄, C₆ = 1-16 pfD1 = UM 9415 PIN diode L1 = 92 nh

[0049]FIG. 7 is an electrical schematic diagram for surface coil 44. Inaddition to passive blocking networks 96, loop capacitances and a 50-Ohmlattice balun 100, the surface coil 44 includes a network 94 forisolating the counter-rotating loops of the surface coil 44. Componentvalues for the elements shown in FIG. 7 are as follows: Network(Correction, Loop Components Balun 100 Decoupling & Match) 94 C₃ = 39pf + 1-16 pf C = 51 pf C1 = 75 pf C₄ = 39 pf + 1-16 pf L = 142 nh L1 =88 nh C₁ = 56 pf C2 = 75 pf C₂ = 47 pf L2 = 88 nh C₅ = 47 pf L3 = 4.7 uhC₆ = 56 pf C3 = .01 uf C4 = .01 uf C5 = .01 uf C6 = .01 uf Lcomp = 114mh Diodes = UM 9415 PIN Diode

[0050]FIG. 8 is an electrical schematic diagram for surface coil 54. Anetwork 94 is included for isolating the co-rotating loops of thesurface coil 54. Component values for the elements shown in FIG. 8 areas follows: Network (Correction, Loop Components Balun 100 Decoupling &Match) 94 C₁ = 47 pf C = 51 pf C1 = 56 pf C₂ = 47 pf L = 142 nh L1 = 110nh C₃ = 47 pf C2 = 56 pf C₄ = 47 pf L2 = 110 nh L3 = 4.7 uh Ccomp = 43pf C3 = N/A C4 = N/A C5 = .01 uf C6 = .01 uf Ctune = 39 pf + 1-16 pfDiodes = UM 9415 PIN Diode

[0051]FIG. 9 is an electrical schematic diagram for surface coil 45,which includes two counter-rotating loops 45 a and 45 b. A network 94 isincluded for isolating the loops. Component values for the elementsshown in FIG. 9 are as follows: Network (Correction, Loon ComponentsBalun 100 Decoupling & Match) 94 C₁ = 75 pf C = 51 pf C1 = 91 pf C₂ = 68pf + 1-16 pf L = 142 nh L1 = 88 nh C₃ = 68 pf + 1-16 pf C2 = 91 pf C₄ =75 pf L2 = 88 nh L3 = 4.7 uh Diodes = UM 9415 PIN Diode C3 = .01 uf C4 =.01 uf C5 = .01 uf C6 = .01 uf Lcomp = 198 nh

[0052]FIG. 10 is an electrical schematic diagram for surface coil 55,which includes two co-rotating loops 55 a and 55 b. Component values forthe elements shown in FIG. 10 are as follows: Network (Correction, LoopComponents Balun 100 Decoupling & Match) 94 C₁ = 43 pf C = 51 pf C1 =102 pf C₂ = 43 pf L = 142 nh L1 = 75 nh C2 = 102 pf L2 = 72 nh L3 = 4.7uh Ccomp = 30 pf + 1-16 pf C3 = 30 pf C4 = 30 pf C5 = .01 uf C6 = .01 ufCtune = 43 pf + 1-16 pf Diodes = UM 9415 PIN Diode

[0053]FIG. 11 is an electrical schematic diagram for surface coils 46and 47. Since the surface coils 46 and 47 may be used in eitherbilateral mode (both on) or unilateral mode (one on, the other off),each of the surface coils has a 50 Ohm output 100 and a PIN diodeswitch, D,. Component values for the elements shown in FIG. 11 are asfollows: Network (Correction, Loop Components Balun Decoupling & Match)94 C₁ = 41 pf C = 51 pf C match 47 = 120 pf C₂ = 51 pf + 1-16 pf L = 142nh C block 47 = 62 pf C₃ = 51 pf + 1-16 pf L block 47 = 142 nh C₄ = 41pf L ISO 47 = 74 nh C match 46 = 91 pf C block 46 = 62 pf L block 46 =142 nh L ISO 46 = 92 nh Diodes, D₁ = UM 9415 PIN Diode

[0054]FIG. 12 is an electrical schematic diagram for surface coils 56and 57. Like the surface coils 46 and 47, the surface coils 56 and 57may be used in either bilateral or unilateral mode. A low-pass phaseshift and matching network 102 couples the balun 100 to the network 94for each coil 56 and 57. Component values for the elements shown in FIG.12 are as follows: Loop Components Balun 100 Low Pass Match 102 C₁ = 56pf C = 51 pf C = 24 pf C₂ = 68 L = 142 nh L = 258 nh C₃ = 75 pf + 1-16pf C₄ = 75 pf + 1-16 pf C₅ = 68 pf C₆ = 56 pf

[0055] Decoupling & Match) 94 C1 = 103 pf L1 = 86 nh C2 = 103 pf L2 = 86nh C3 = 33 pf L3 = 198 nh L4 = 198 nh

[0056] Diodes=UM 9415 PIN Diode

[0057]FIG. 13 is an electrical schematic diagram for surface coils 48and 49. The surface coils 48 and 49 may be used in either bilateral orunilateral mode. Component values for the elements shown in FIG. 13 areas follows: Network (Correction, Loop Components Balun 100 Decoupling &Match) 94 C₁ = 75 pf C = 51 pf C match 49 = 270 pf C₂ = 82 pf L = 142 nhC block 49 = 62 pf C₃ = 75 pf + 1-16 pf L block 49 142 nh C₄ = 75 pf +1-16 pf L ISO 49 = 74 nh C₅ = 82 pf C₆ = 75 pf C match 48 = 240 pf Cblock 48 = 62 pf L block 48 = 142 nh L ISO 48 = 92 nh Diodes = UM 9415PIN Diode

[0058]FIG. 14 is an electrical schematic diagram for surface coils 58and 59. The surface coils 58 and 59 may be used in either bilateral orunilateral mode. A low-pass phase shift and matching network 102 couplesthe balun 100 to the network 94 for each coil 58 and 59. Componentvalues for the elements shown in FIG. 14 are as follows: Loop ComponentsBalun 100 Low Pass Matching 102 C₁ = 91 pf C = 51 pf C = 24 pf C₂ = 91pf L = 142 nh L = 258 nh C₃ = 68 pf + 1-16 pf C₄ = 68 pf + 1-16 pf C₅ =91 pf C₆ = 91 pf Network (Correction, Decoupling & Match) 94 C1 = 130 pfL1 = 65 nh C2 = 130 pf L2 = 65 nh C3 = 47 pf L3 = 142 nh L4 = 142 nhDiodes = UM 9415 PIN Diode

[0059]FIG. 15 is an electrical schematic diagram for surface coils 50and 51. The surface coils 50 and 51 may be used in either bilateral orunilateral mode. A low pass phase shift and matching network 102 couplesthe balun 100 to the network 94 for each coil 50 and 51. Componentvalues for the elements shown in FIG. 15 are as follows: Loop ComponentsBalun 100 Low Pass Matching 102 C₁ = 68 pf C = 51 pf C = 24 pf C₂ = 68pf L = 142 nh L = 258 nh C₃ = 62 pf + 1-16 pf C₄ = 62 pf + 1-16 pf C₅ =68 pf C₆ = 68 pf Network (Correction, Decoupling & Match) 94 C1 = 130 pfL1 = 68 nh C2 = 130 pf L2 = 68 nh C3 = 47 pf L3 = 114 nh L4 = 114 nhDiodes = UM 9415 PIN Diode

[0060]FIG. 16 is an electrical schematic diagram for surface coils 60and 61. The surface coils 60 and 61 may be used in either bilateral orunilateral mode. A low-pass phase shift and matching network 102 coupleseach balun 100 to the network 94. Component values for the elementsshown in FIG. 16 are as follows: Loop Components Balun 100 Low PassMatching 102 C₁ = 110 pf C = 51 pf C = 24 pf C₂ = 91 pf L = 142 nh L =258 nh C₃ = 62 pf + 1-16 pf C₄ = 62 pf + 1-16 pf C₅ = 91 pf C₆ = 110 pfNetwork (Correction, Decoupling & Match) 94 C1 = 180 pf L1 = 45 nh C2 =180 pf L2 = 45 nh C3 = 100 pf C4 = 100 pf L3 = 44 nh Diodes = UM 9415Diode

[0061] As shown in FIGS. 11 through 16, the surface coil pairs in thelower leg portion of the peripheral vascular array 40 include anisolation network which operates to cancel the coupling due to mutualinductance. While the mutual inductance could have alternatively beenreduced by overlapping the adjacent coils in the surface coil pairs(46,47), (48, 49), (50,51), (56, 57), (58, 59) and (60, 61), the use ofthe isolation network is preferable because it allows the loops in thecoil pairs to be significantly smaller. As a consequence, the signal-tonoise ratio is improved. In addition, by using smaller separated coilswith an isolation network rather than overlapping larger coils, aliasingeffects are reduced. Moreover, the isolation networks allow the surfacecoil pairs to operate as either a single loop (e.g. in an unilateralmode) or as combined counter-rotating and co-rotating pairs (e.g. inbilateral mode).

[0062] The surface coil array 10 described above may be connected towell known scanners to obtain a variety of images. The coils aretypically connected to signal receivers in the scanners via preamplifierinputs. The number of signal receivers in a scanner is preferably keptsmall due to the cost of the signal receiver. For example, one knownscanner uses four signal receivers which may receive signals from asmany as eight preamplifier inputs. As discussed above, a preferredsurface coil array 10 may contain as many as 20 coils 12. The coilinterface 20 illustrated in FIG. 1 may be used to select groups of coilsfrom the N surface array coils 12(l) through 12(N) to connect to the Ppreamplifier inputs to M signal receivers where N is greater than both Mand P.

[0063] The coil interface 20 in FIG. 1 includes a switch 22 and a logiccircuit 24 for controlling the state of the switch 22. The logic circuit24 controls the state of the switch 22 according to configurations orgroups of coils 12 that are combined to produce images targetingspecific areas of the body. The configurations may be specified bysignals at a coil select input 28 from the scanner 30 in response touser input. Alternatively, signals may be generated by otheruser-accessible sources, such as dip-switches or other suitable devicesthat may be connected to the interface by a cable, which may be electricor fiber optic. An infrared connection may also be used for remotecontrol selection of coil groups.

[0064]FIG. 17 is a block diagram of a scanner 300 and a 20-coil surfacecoil array 120 that uses a coil interface 200 according to a preferredembodiment. The coil interface 200 in FIG. 17 includes atransmit/receive (“TR”) driver 130, an RF switch array 220 and controllogic 240, and interfaces the surface coil array 120 to the scanner 300at a pre-amp array 303, which is internal to the scanner 300. Thepre-amp array 303 connects to receivers 301 via a switching and routingcircuit 302. The surface coil array 120 may, for example, be arranged inthe form of the peripheral vascular array 40 described above withreference to FIG. 2A. The scanner 300 in a preferred embodiment is aSigna system with the phased array option from General Electric, asdescribed above.

[0065] As shown in FIG. 17, the control logic circuitry 240 includes anRF switch controller 250, a TR driver controller 260 and error checkingcircuitry 265. The control logic circuitry 240 receives a coil selectinput 270 from the scanner 300 as the coil group selector input. Thecoil select signals at the coil select input are a four-bit digital wordwith DC voltage levels providing binary logic levels. The coil selectinput 270 may be coupled to the lines that connect to the coilsthemselves, and may thereafter, have some RF components. Inductors L₁₋₄filter out any RF components so that a DC signal is received by thecontrol logic circuitry 240.

[0066] The control logic circuitry 240 also receives a mode signal froma mode switch 242. The mode switch 242 allows a user to select aunilateral right, a unilateral left or a bilateral imaging mode. Themodes are useful where right and left coils may be combined in thebilateral mode to obtain an image with a wider field of view, orisolated in the right or left modes to isolate a selected side. Oneadvantage of isolating a selected side is that an improved signal tonoise ratio is obtained thereby providing an image with a higherresolution.

[0067] The RF switch controller 250 uses the coil select input 270 andthe mode signal from the switch 242 to select RF switch control lines280. The selected RF switch control lines 280 enable RF switches in theRF switch array 220, which connect selected coils from the surface coilarray 120, to couple image signals from the selected coils to the inputs304 of the scanner 300.

[0068] The coil select input 270 is preferably coupled to the TR drivercontroller 260. The TR driver controller 260 uses the coil select input270 to determine which coils are going to be used for imaging. The TRdriver controller 260 outputs signals on the coil enable inputs 261 toenable the coils that are to be used for imaging and disable theremaining coils. The coil select input 270 advantageously permits theuser to select different coil configurations without any scan roomintervention.

[0069] The coil select input 270 may, for example, be a four-bit wordgenerated by the scanner 300 when the user enters a request for imagesrequiring a certain coil configuration. The user's request may beentered at a console (not shown). Alternatively, an input that isseparate from the scanner 300 may be used. For example, a separatekeypad may be used to input signals that designate a desired coilcombination. Other inputs include, DIP switches, toggle switches, etc.To enter the request, the user may enter the four-bit word itself, agroup identifier, a request for an image of a body part, or any othersuitable input that the scanner 300 is programmed to understand as agroup of coils or sequence of coil groups. The four-bit word in apreferred embodiment actually has the dual function of communicating aTransmit/Receive state to the coils as well as providing a groupconfiguration input. When the scanner 300 generates a +5v. signal on allcoil select input lines 270, the scanner is in the Transmit state, inwhich case the remote coil is active and all of the receive coils (i.e.the coils in the surface coil array 120) are preferably activelydisabled, such as by the PIN diode switches shown in FIGS. 5 through 16for the peripheral vascular array 40, and not connected to thepreamplifiers 303. When not all of the coil select input lines 270 areat +5v., the coils connected to the preamplifiers 303 are selected inaccordance with the four-bit word.

[0070] The logic control circuitry 240 includes an error checkingcontroller 265 for sensing error conditions in the coils 120 or coilinterface 200. The error checking controller receiving error states fromthe TR driver 130 or error state lines 262, which are described belowwith reference to FIG. 19. The error checking controller 265 may alsogenerate fault conditions on transistors 267(l)-267(4) to check forerrors. Transistors 267(l)-267(4) are normally in a non-conductingstate. When switched to a conducting state. the coil select input lines270 may be put in an error checking made by switching the states of thelines to a logic 0 or logic 1 detect a specific fault. Conditions suchas coil diode shorts, diode opens, DC power failure and TR driverfailure may be sensed on lines 262 in response to the fault generated.

[0071] In a preferred coil interface 200, the 20 surface coils aregrouped into groups of coils that produce specific, useful images. FIG.2A illustrates the posterior and anterior coils 40 as COIL1 42, COIL243, COIL3 52, COIL4 53, COIL5 44, COIL6 54, COIL7 45, COIL8 55, COIL946, COIL10 47, COIL11 56, COIL12 57, COIL13 48, COIL14 49, COIL15 58,COIL16 59, COIL17 50, COIL18 51, COIL19 60 and COIL20 61. FIG. 18 is acoil group table 400 that describes groups of coils 402, a mode switchsetting 404, coils selected for a group at 406, and comments 408describing an image obtained by selecting the group of coils identifiedin each row of the table.

[0072] As shown in the coil group table 400, the Group 1 coils COIL1 42,COIL2 43, COIL3 52 and COIL4 53 are selected in order to obtain an imageof vasculature from the renal arteries to the bifurcation. The functionof the mode switch 242 (in FIG. 17) is illustrated by comparing Group 5with Groups 8 and 10. In Group 5, the mode switch 242 is set to“Bilateral” as indicated in column 404. The coils selected in Group 5are COIL13 48, COIL14 49, COIL15 58, COIL16 59, COIL17 50, COIL18 51,COIL19 60 and COIL20 61. The signals from these coils are combined inpairs as shown in FIG. 18 to provide an image of both the right and leftfeet By setting the mode switch 242 to “Right” (in FIG. 17) andselecting COIL13 48, COIL15 58, COIL17 50 and COIL19 60 as shown forGroup 8, images of only the right foot and ankle are provided.

[0073] It is to be appreciated by one of ordinary skill in the art thatFIGS. 2A and 18 illustrate one example of a configuration of surfacecoils that may be used with the coil interface of the present invention.With changes to the coil interface that are within the ability of one ofordinary skill in the art, any number of coils may be connected to alimited number of inputs according to functionally defined groups.

[0074] Referring to FIG. 17, the coil select input 270 is used by the TRdriver controller 260 to enable coils that are to receive an imagesignal and to disable all other coils. The TR driver controller 260determines which coils are to be used according to the group identifiedby the control select input 270. For each coil to be used, a coil enablesignal is output on a corresponding coil enable input 261. The coilenable signal switches the TR driver 130 to the enable state, whichpermits current to flow through the PIN diode of the selected coil. TheTR driver 130 maintains coils that do not receive a coil enable signalin a disabled state to prevent noise generated by coils from which animage signal is not desired. An advantage of enabling only coils thatwill receive image signals and disable all of the coils is that thesignal to noise ratio is improved.

[0075] In a preferred embodiment, the TR driver 130 includes a coildriver 132 for each coil (COIL1, COIL2, COIL3, COIL4) in the surfacecoil array 120 as shown in FIG. 19. The coils 120(1)-120(4) are shown inFIG. 19 with the PIN diode used to drive the coil and enable an image RFsignal to be input at the RF switch array 220. The coil drivers132(a)-(d) are arranged in a totem-pole configuration 134 and suppliedby a current source 136. In FIG. 19, only four coil drivers 132 areshown in a stack. Any number of coil drivers 132 may be connected in astack. The number of coil drivers 132 in a stack is preferably theapproximate maximum number of coils that can be simultaneously driven bythe power supply.

[0076] Each coil driver 132 includes a differential switch 140(a) inwhich the gates of two FETs 142(a), 144(a) of opposite type are drivenby the coil enable input 261(a). When the coil enable input 261(a)receives a coil disable signal (logic 1, −15v.), the first FET 142(a)provides a current path 146 for current away from the coil 120(1). Whenthe coil enable input 261(a) receives a coil enable signal (logic 0,5v.), the second FET 144 provides a current path 147 for current throughthe coil 120(1).

[0077] One advantage of using the totem pole configuration shown in FIG.19 is that the number of coils that can be driven at one time ismaximized. For example, if the PIN diodes in the coils are driven by a−10v (−v=10v.) power supply that can provide up to 800mA, the powersupply may sag to about −8.5 due to wiring losses. Using the totem poleconfiguration, and assuming about a 0.9v. drop per diode, 9 diodes maybe simultaneously driven by the single current source 136. If each coildriver 132(a)-(d) and coil diode were to be driven by the power supplyin parallel, four or fewer diodes may be driven simultaneously inparallel. Although power supplies may vary according to the type NMRscanner used, the advantages offered by the totem pole configuration,particularly that of maximizing the number of coils driver are stillavailable.

[0078] Another advantage of the totem pole configuration is thaterror-checking functions may be incorporated into the coil interface bysensing the state of the voltage levels at selected points in the coildrivers 132(a)-(d). In a preferred embodiment, at least four errorconditions may be sensed: coil diode open, coil diode short, transistor(FET) open, transistor (FET) short.

[0079] The error conditions in a preferred embodiment may be sensed bygenerating fault conditions as described above with reference to FIG.17, and by using an upper error switch 145(a)(l) and a lower errorswitch 145(a)(2) each having digital outputs to the logic circuit262(a)(1) and 262(a)(2), respectively. The FET transistor 142(a) and FETtransistor 144(a) must be in opposite states at all times. If outputs262(a)(1) and 262(s)(2) of the upper and lower error switches 145(a)(1),145(a)(2) are in the same state, a diode open or a diode short issensed.

[0080] For example, if coil enable input 261(a) has an enable signal,the FET transistor 144(a) is in the ‘ON’ state thereby providing currentto the diode in coil1; and the FET transistor 142(a) is in the ‘OFF’state. When the FET transistor 144(a) is ‘ON’, the lower error switch145(a)(2) is ‘ON’ and when the FET transistor 142(a) is ‘OFF’, the uppererror switch 145(a)(2) is ‘OFF’. If the coil PIN diode for COIL1 isopen, the lower error switch 147(a) will remain in the ‘OFF’ state evenwhen an enable signal (5v) is received. Both 262(a)(1) and 262(a)(2)outputs will be sensed in the low state by the logic circuit 240. In apreferred embodiment, the scanning will be aborted when an error isdetected A window comparator 151 is used in a preferred embodiment todetect transistor open or transistor short conditions when outputs Q₀and Q₁. are in opposite states and therefore appear normal. If atransistor (such as 142(a), 144(a), etc.) is open, not enough current isbeing drawn through R_(error) (2.8 ohms). The window comparator 151 willdetect a voltage at 153 that is greater than V_(ul). If a transistor isshorted, too much current will be detected by the window comparator whenthe voltage at 153 is lower than V_(ll).

[0081] The groups in the coil group table 400 in FIG. 18 may be selectedusing the RF switch array 220 in FIG. 17. FIG. 20 shows animplementation of the RF switch array 220. The RF switch array 220 inFIG. 20 includes RF switches SW1-SW20 and RF combiners CMB1-CMB6connected in the configuration shown. The RF switches SW1-SW20 areenabled by control inputs 280. Control inputs 280 each include one ormore control lines that are controlled by the logic circuit 240 asdescribed below with reference to FIGS. 23A and 23B.

[0082] The configuration of RF switches SW1-SW20 and combiners CMB1-CMB6determines the surface coils to be selected for input of the imagesignal according to the groups selected from the coil selection input270 and mode switch 242 (in FIG. 17). The RF switches SW1-SW20 mayinclude the switches illustrated in FIGS. 21A-21C, as well as variationsof the switches in FIGS. 21A-21C.

[0083] The switches in FIGS. 21A-21C use PIN diodes as the preferredswitching element. PIN diodes are fast, non-magnetic switches that mayhave a resistance on the order of a few ohms in the ‘on’ state.

[0084]FIG. 21A illustrates a single RF switch having one control input450 controlling a single PIN diode D1. The RF switch input 460 iscoupled to a coil with an RF imaging signal that may include a DCvoltage. When the control input 450 is set to a voltage that issufficiently positive to forward bias the PIN diode D1, the diode D1switches to a conducting state and behaves like a resistor. The diode D1conducts the RF imaging signal at the input through capacitors C1 andC2, which block any DC components, to RF switch output 470. Theinductors L1and L2 filter out the RF signal from the control input 450and from ground allowing the signal to be coupled to the output 470.

[0085]FIG. 21B illustrates an RF switch having a single input 460 thatcan switch to either of two outputs 470 a and 470 b. The RF signalcoupled to RF switch input 460 is output to output 470 a when controlinput 450 a forward biases diode D1 and to output 470 b when controloutput 450 b forward biases diode D2. In one variation of the switch inFIG. 20B, multiple PIN Diodes D1, D2 may share the same control input450.

[0086]FIG. 21C illustrates an RF switch having multiple inputs and asingle output. Each input 460 a, 460 b, 460 c couples to a respectivediode D1, D2, D3. The diodes D1, D2, D3 are connected to a common output470. When the control input 450 a, 450 b, or 450 c corresponding to thediode D1, D2 or D3 forward biases the diode, the signal at the input iscoupled to the RF output 470. Multiple PIN Diodes D1, D2 may share thesame control input 450.

[0087] FIGS. 22A-22B are schematic representations of RF switch array220 illustrating the components in RF switches SWI-SW20 and combinersCMB1-CMB6. RF switches SW1, SW2 and SW5 are shown with RF switch controlinputs 450, RF switch inputs 460 and RF switch outputs 470 labeledaccording to the conventions in FIGS. 21 A-21 C. The RF control inputs450 for each switch interface to the RFS1-RFS33 lines on ports P2 andP4. Ports P2 and P4 in a preferred embodiment interface to the controllogic 240 which includes circuitry for selecting coils.

[0088]FIGS. 22A and 22B illustrate the control of RF switches byselectively enabling RFS1-RFS33. For example, if the coil select inputdesignates a coil group, the RF switch controller 250 determines whichRF switch or switcher are to be enabled. The coil enable signal (i.e.logic or +5V in a preferred embodiment) is output by the RF switchcontroller 266 in the control logic circuitry 240 on RFS03. The SVsignal forward biases diode D1. With Diode D1 forward biased, the RFsignal at coil 8 is output by SW1 at RF switch output 470.

[0089] As shown in FIG. 22A, the output 470 of switch SW1 is coupled tobit A of the coil select input 270. Capacitor C2 blocks the DC voltagesignal applied to the output 470 when the coil select input 270 selectsa coil group. By blocking DC signals capacitor C2 permits Diode D1 to beforward biased.

[0090] The combiners CMB1-CMB6 are typical RF signal combiners such asWilkerson combiners that are used to combine RF imaging signals from twoseparate coils. For example, Groups 4 and 5 in FIG. 6 use signals thatare a combination of RF imaging signals from different coils.

[0091] The preamplifiers 303 in the scanner 300 are generally sensitiveto source impedance, which in FIG. 17, for example, is dependent uponthe RF electrical characteristics of the coil interface 200 and thesurface coil array 120. This sensitivity is typically quantified interms of the noise figure of the preamplifiers 303.

[0092] In accordance with a preferred embodiment of the presentinvention, the RF design of the coil interface 200 and the surface coilarray 120 minimizes the effect of this sensitivity by presenting thepreamplifiers 303 with substantially the same source impedance,regardless of the mode of operation (left, right or bilateral) ofsurface coil array 120. This may be accomplished by setting theelectrical length of the entire transmission path from the surface coilto the preamplifier 250 to be equal to an odd multiple ofquarter-wavelengths. Since the combiners CMB1-CMB6 in the RF switch 220are in the transmission path for bilateral imaging and out of thetransmission path for unilateral imaging, the bilateral imagingtransmission path includes additional phase delay from the combinersCMB1-CMB6, which may be compensated for by using a phase advance Tnetwork, in series with the combiners CMB1-CMB6. In a similar manner, aπ network may be used to adjust the electrical length of the unilateralimaging transmission path. The implementations of T and π phase-shiftingnetworks are well known to those skilled in the art.

[0093] The selection of coils for the input of RF imaging signals isaccomplished by the control logic 240 which uses the coil select input270 and mode select switch 242 (in FIG. 17) to output control signals onthe RFS1-RFS233 lines 280. FIG. 23A illustrates a programmable logicdevice (PLD) U93 used to output control signals RFS1-RFS33 in responseto the coil select input 270 and mode switch 242. The PLD U93 outputscontrol signals at outputs PLD101-PLD133. FIG. 23B is a table thatillustrates the RFSxx signal that corresponds to the PLDXxx signals inFIG. 23A. FIG. 23B also illustrates the states of coil select input 270and the states of mode select switch 242. The states of the RSFxx linesat 280 corresponding to the states of the coil select input 270 and modeselect switch 242 are also provided in FIG. 23B. A state of ‘0’ forRSFxx indicates that the corresponding switch is enabled. The logical‘0’ in a preferred embodiment is set at 5v. while the logical ‘1’ is setat -15v. The state of ‘0 ’therefore forward biases the PIN diode at thecontrol input of the switch corresponding to the specified RSFxx line.

[0094]FIG. 23C shows the states of the coil enable inputs 261 accordingto the coil select input 270 and mode switch 242. FIG. 23C illustratesthe coils selected for various states of the coil select input 270 andthe mode select switch 242. By referring to FIGS. 23B and 23C, one ofordinary skill in the art can determine the combinations of RF switchesSWI-SW20 and coil enable inputs 261 used to select coils for the desiredcoil groups.

[0095] In accordance with a preferred method for imaging the peripheralvasculature with the peripheral vascular array 40, a combination ofcontrast study and time-of-flight imaging is utilized. Generallyspeaking, the use of a contrast agent, such as Gadolinium, will improveimage quality and reduce inspection times. Such contrast agents are,however, relatively expensive and the imaging of the entire peripheralvasculature would require a substantial amount of the contrast agent.The method therefore utilizes a contrast agent for imaging only thoseareas where time-of-flight imaging is difficult.

[0096] In particular, the method includes the step of performing acontrast study of the renal arteries and the abdominal bifurcation byacquiring image information using surface coils 42, 43, 52 and 53. Thetiming of image acquisition is coordinated with the injection of thecontrast agent in any known manner. Time-of-flight imaging is thenutilized to acquire image information from the vasculature in the legs,using, for example, surface coils 44 through 49 and 54 through 59.Images of the feet may be obtained using either the contrast study orthe time-of-flight technique by acquiring image information from surfacecoils 50, 51, 60 and 61.

[0097] In the alternative, images of the peripheral vasculature may beobtained using the peripheral vascular array 40 with only time-of-flightimaging. This technique, however, may require longer examination timesdue to the difficulty of using time-of-flight imaging to acquire imageinformation in structures having sagittal plane blood flow, such as therenal arteries.

[0098] In accordance with another preferred method, the peripheralvascular array 40 acquires successive adjacent axial images in timedrelation to the progression of a bolus of contrast agent through theperipheral vasculature. This is made possible by the large area coveredby the peripheral vascular array 40, which allows images from the renalarteries through the feet to be obtained without repositioning the array40.

[0099] While the invention has been described in conjunction withpresently preferred embodiments of the invention, persons of ordinaryskill in the art will appreciate that variations may be made withoutdeparture from the scope and spirit of the invention. The true scope andspirit of the invention is defined by the appended claims, interpretedin light of the foregoing description.

We claim:
 1. A coil interface in an imaging system, comprising, incombination: N image signal inputs; a plurality of coil switchesconnected to a plurality of said N image signal inputs; circuitry forselecting a group of said N image signal inputs by enabling a selectedgroup of said plurality of switches in response to a group selectorinput; and M image signal outputs, at least one of said image signaloutputs being connected to a selected group of N image signal inputswhen said selected group of said plurality of switches is enabled, saidimage signal outputs being operable to receive said image signals when Nis greater than M.
 2. The coil interface of claim 1 further comprising ppreamplifiers, at least one of which is operatively connected at apreamplifier input to said selected group of N image signal inputs whensaid selected group of said plurality of coil switches is selected, saidpreamplifiers having preamplifier outputs connected to correspondingimage signal receivers, wherein p is greater than or equal to M.
 3. Thecoil interface of claim 2 further comprising: a plurality ofpreamplifier switches, each of said preamplifier switches having atleast one signal input connected via selected coil switches to said Nimage signal inputs and signal outputs connected to correspondingpreamplifiers.
 4. The coil interface of claim 1 wherein said pluralityof coil switches includes an RF switch comprising: at least one RFswitch input each coupled to a corresponding one of said N image signalinputs; and at least one PIN diode corresponding to said at least one RFswitch input, said at least one PIN diode coupled at the anode to thecorresponding one of said at least one RF switch input and coupled atthe cathode to an RF switch output, said PIN diodes operative to outputthe image signal from said one of said N image signal inputs in aforward bias state and operative to block said RF signal in a reversebias state.
 5. The coil interface of claim 4 further comprising at leastone control input coupled to a corresponding one of said at least onePIN diode to set said PIN diode in the forward bias state when saidcontrol input receives an enable signal and in the reverse bias statewhen said control input receives a disable signal.
 6. The coil interfaceof claim 1 wherein said plurality of coil switches includes an RF switchcomprising: an RF switch input coupled to a corresponding one of said Nimage signal inputs; and a plurality of PIN diodes coupled at thecathode of each PIN diode to a corresponding plurality of RF switchoutputs and coupled at the anode to the RF switch input, said PIN diodesoperative to output the image signal from said one of said N imagesignal inputs in a forward bias state and operative to block said RFsignal in a reverse bias state.
 7. The coil interface of claim 6 furthercomprising a plurality of control inputs coupled to set a correspondingPIN diode in the forward bias state when said control input receives anenable signal and in the reverse bias state when said control inputreceives a disable signal.
 8. The coil interface of claim 1 wherein saidplurality of coil switches includes an RF switch comprising: a pluralityof RF switch Inputs each coupled to a corresponding one of said N imagesignal inputs; and a plurality of PIN diodes coupled at the cathode ofeach PIN diode to a corresponding plurality of RF switch outputs andcoupled at the anode to a corresponding one of the plurality of RFswitch inputs, said PIN diodes operative to output the image signal fromsaid one of said N image signal inputs in a forward bias state andoperative to block said RF signal in a reverse bias state.
 9. The coilinterface of claim 8 further comprising a plurality of control inputscoupled to set a corresponding PIN diode in the forward bias state whensaid control input receives an enable signal and in the reverse biasstate when said control input receives a disable signal.
 10. The coilinterface of claim 1 further comprising at least one RF combiner forcombining the image signal from one of said N image signal inputs withthe image signal from another one of said N image signal inputs.
 11. Aninterface for connecting a surface coil array to an NMR scanner,comprising: a first number, N, of RF inputs, the RF inputs beingarranged to receive signals from the surface coil array; an RF switchcircuit coupled to the RF inputs, said RF switch circuit having a secondnumber, M, of RF outputs, wherein the RF switch circuit comprises afirst plurality of RF switches coupled to the RF inputs, a secondplurality of RF combiners coupled to receive outputs from a subset ofthe first plurality of RF switches, and a third plurality of RF switchescoupled to receive outputs from the second plurality of RF combiners;and a control logic circuit coupled to the RF switch circuit, whereinthe control logic circuit receives a coil select signal and controls astate of the first plurality of RF switches and the third plurality ofRF switches in response to the coil select signal.
 12. An interface asclaimed in claim 11, wherein the coil select signal is provided by 5 theNMR scanner.
 13. An interface as claimed in claim 11, wherein the firstnumber, N, is greater than the second number, M.
 14. An interface asclaimed in claim 13, wherein N=20 and M=8.
 15. An interface as claimedin claim 11, wherein the subset of the first plurality of RF switcheseach have one input coupled to the RF inputs and two outputs, wherein asignal at the RF switch input is coupled to one of the two outputs inaccordance with a signal from the control logic circuit.
 16. Aninterface as claimed in claim 15, wherein one of the two outputs of theRF switch is coupled to one of the second plurality of RF combiners andthe other output of the RF switch is coupled to an input to one of thethird plurality of RF switches.
 17. An interface as claimed in claim 11,wherein each RF combiner in the second plurality of RF combiners isoperable to combine a pair of RF inputs from the surface coil array intoa single RF output.
 18. An interface as claimed in claim 17, wherein thesingle RF output is coupled by one of the third plurality of RF switchesto the NMR scanner.
 19. An interface as claimed in claim 18, whereinfour pairs of RF inputs are combined into four outputs by four REcombiners, the four outputs being coupled to the NMR scanner.
 20. Aninterface as claimed in claim 11, further comprising a transmit/receivebias circuit coupled between the RF inputs and the RF switch, whereinthe transmit/receive bias circuit is coupled to and controlled by thecontrol logic circuit to actively disable selected coils from thesurface coil array.